Additional scientists can be obtained as rewards for completing certain missions, (mainly abduction missions,) monthly rewards for having satellites in Europe, North America, South America and Africa (the more satellites, the more scientists you will receive each month,) and from the occasional council request. Once you have a total of 80 scientists, the achievement will unlock. This will most likely come naturally towards the later stages of the game
TIP: If the achievement does not unlock naturally over the course of the game, start a new game on easy and focus on completing tasks/launching satellites that will net you the most scientists. If given the option between adding scientists or engineers, always choose scientists.